POWER INFRASTRUCTURE
HV trip off (EPO) button
HV dc tripping battery
Inter-posing relay
11,000 v HV network
CB (HV)
Figure 2. Restricted earth fault (REF): HV-LV inter-tripping arrangement.
3. Transformer selection and earthing arrangements
n Key issues: High losses, unsuitable or missing earthing and poor segregation of HV and LV Systems In addition, there is a lack of transformer protection systems i.e. restricted earth fault protection and/or over- temperature protection. The integrity of the transformer star-point earthing (grounding) a transformer is essential for ensuring safety by limiting touch voltages during faults, providing a return path for fault current, allowing protection devices to operate, stabilising system voltage during unbalanced conditions or surge events, and reducing electromagnetic interference in communication/ control cables. In addition, poor levels on transformer protection are evident in many cases. Transformer selection must consider technical performance and clinical impact. In addition, the Transformer is the handshake between the HV & LV systems. Therefore, HV/LV Protection systems are required which should include restricted earth fault (REF) protection which should be arranged as shown in Figure 2. Note: HV dc system used for tripping circuit. Where hermetically sealed transformers are specified
and high-pressure relays are fitted, then these also should trip the HV breaker using the HV dc system for the tripping circuit.
Liquid filled transformers (ON & KNAN typically)
should also have high temperature (HT) monitoring and high temperature trip (HTT) protection units fitted to protect these valuable assets. In the case of transformer high temperature protection,
these should trip the LV breaker and not the HV breaker as high temperature is load driven with typical arrangement as seen in Figure 3. n Recommendation: Use Tier 2 or Eco Design transformers. Employ TN-S earthing for essential supplies. Ensure high integrity start point earthing installed. Ensure appropriated HV/LV inter-tripping & transformer protection system systems are in place to meet Electricity at Work Regulations 1989.
4. Main LV intake switchboard n Key issues: Frequent failure points include lack of Form
TR TC
Trip coil Ref
Restricted earth fault relay
400 v LV network
Liquid filled transformers (ON & KNAN typically) should also have High Temperature (HT) monitoring & High Temperature Trip (HTT) protection units fitted to protect these valuable assets.
4 or 5 internal separation and outdated protection interfaces.
There is a large legacy of LV switchboards which are non IP2X especially on cable termination chambers where exposed Live copper conductors are present. In addition, incorrect protection on main incomers such as unrestricted earth fault protection where an earth fault in the system incorrectly trips the main incoming breaker. n Recommendation: To enable safe working on main LV switchboards, they should be designed for Form 4 (Type 6 Minimum) ingress protection. Protection must allow for upstream HV discrimination. Replace legacy systems lacking arc containment or proper segregation.
5. HV/LV protection and inter-tripping systems
Breakers, especially air circuit breakers (ACBs) and moulded case circuit breakers (MCCBs) are dispatched from the factory with their protection units set to a minimum as factory default settings. Unless adjusted during commissioning, nuisances tripping can occur. DC tripping/ closing batteries are common failure points. n Recommendation: Use 24/30V dual rectifier battery systems with BMS monitoring. Implement local and remote alarm annunciation. Test battery banks quarterly. Note: there should be separate HV & LV tripping
HT
Note: Core shown only for clarity
Liquid
HTT HT
Figure 3. Transformer high temperature: monitoring and protection-tripping arrangement.
11,000 v HV network CB (HV) LV dc HTT
HTT HT
Core
Trip coil Core
TC HTT
HT Inter-posing relay
400 v LV network
BMS
tripping battery
60 Health Estate Journal January 2026
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